Isolation trap



April 22, 1958 D. ALPERT 2,831,549

ISOLATION TRAP Filed Aug. s1, 1954 |5,` Vacuum Evocuoed f" Pump ChamberI3 Fig. l

Fig. 2.

Io-o El INVENTOR Daniel Alpert.

nited States Patent ISOLATION TRAP Daniei Alpert, Churchill, Pa.,assigner to Westinghouse Electric Corporation, East Pittsburgh, Pa., acorporation of Pennsylvania Application August 31, 1954, Serial No.453,229

9 Claims. (Cl. 183-4) This invention relates to an improved isolationtrap for removing vapors from a gaseous stream.

In accordance with prior art, evacuating systems employing mercury oroil dilfusion pumps have been found to be capable of reducing andmaintaining the pressure of the evacuated system to less than -3 mm.mercury. In order to further reduce the pressure of the system, it hasbeen found necessary to prevent the oil or mercury vapors from diffusingor back-streaming into the system to be evacuated. This has beenaccomplished by interposing a trap in the path between the pump and thesystem to be evacuated. There have been several types of traps employedfor this purpose but they havebeen found to have various limitations andthe pressure obtainable has only been approximately 10*7 mm. mercury,

ln particular, `one embodiment of my invention is directed toward a typeof trap which is commonly called a re-entrant cold trap and is fullydescribed on page 75 of A Manual of Vacuum Practice, by L. H. Martin andR. D. Hill, published by the Melbourne University Press. Essentially,this cold trap comprises an elongated cylindrical envelope of glass withan axially located glass inlet tubular member disposed partially withinthe envelope and sealed through the wall of the upper portion of theenvelope. The tubular member is open on both ends, the lower portion ofwhich extends into the lower region of the interior of the envelope andthe upper portion is connected to the pumping system. An outlet openingis also provided through the wall on the upper portion of the envelopewhich is connected to the system to be evacuated. The outer surface ofthe lower portion of the envelope is cooled to a low temperature byimmersion in a suitable refrigerant such as liquid air or solid carbondioxide and acetone. This type trap operates on the theory of condensingthe vapor molecules out onto the cool surface of the interior of theenvelope and the tubular member. in practice it has been found almostimpossible without making an expensive liquid level control system tomaintain the level of refrigerant around the envelope. As a result, adrop in the level of the refrigerant permits the upper portions of thewalls of the envelope and also the surface of the tubular member whichhad previously obtained a low temperature due to the refrigerant to warmup. As a result of this warming up, the vapors which were condensed ontothe cooled surface 1re-evaporate and nd their way into the system tobeevacuated, thus destroying the full effectiveness of the trap. Also, amolecule of condensable vapor may proceed through the trap after a smallnumber of collisions with the walls it the probability of sticking tothe cool surface is not high. That is, if on the average it requiresseveral collisions with the surface before a molecule sticks, thegeometry of a standard re-entrant trap is such that an appreciablenumber of such molecules will go right through the trap.

It has also been foundto be dangerous to utilize liquid air which isessentially liquid oxygen as the refrigerant in that there is a greatdanger of explosionif the envelope should break and the oil vapors comein contact with the ice 2 liquid air. Similar danger is possible even ifliquid N2 is used since it may liquefy some of the O2 from theatmosphere with similar results.

It is, accordingly, an object of my invention to provide an improvedisolation trap for vacuum systems.

It is another object to provide an improved type trap which shall berelatively insensitive to the variation in the level of the refrigerantin which it is immersed.

It is another object to provide an improved type trap which shall avoidany danger of explosion.

It is another object to provide an improved trap that is capable ofoperation in combination with an oil diffusion pump without the use ofany refrigerant.

These and other objects are effected by my invention as will be apparentfrom the following description taken in accordance with the accompanyingdrawing, throughout which like reference characters indicate like parts,and in which:

Figure 1 shows an evacuating system;

Fig. 2 is a perspective view of a re-entrant type cold trap partiallyimmersed within a refrigerating liquid embodying my invention;

Fig. 3 is a straight type trap embodying my invention; and,

Fig. 4 is a cross sectional view taken lV- -1V of Pig. 2.

Referring in detail to Figs. l, 2 and 4, the re-entrant type trap 13shown is comprised of a closed elongated cylindrical container orenvelope 10 of suitable material such as glass. Positioned on the upperportion of the envelope 10 there is provided an outlet pipe or tubing 12which is sealed into the wall of the envelope 10. The outlet tubing 12is connected to the chamber or device 11 to :be evacuated. Centrally oraxially disposed within the envelope 10 is a tubular member 14 of asuitable material such as glass which is sealed to the upper portion ofthe envelope 10. The tubular member 14 is open on both ends and thelower portion disposed within the envelope 10 so that the lower lopeningof the tubular member 14 is positioned in the lower region of theinterior of the enalong the line velope 10. The upper opening of thetubular member` is connected to the pumping system 15. The annularregion or space 16 which is formed between the walls of the tubularmember 14 and the walls of the envelope 10 is filled with a suitableperforated thermal conductive member 1S. In the specic embodiment shown,the member 18 which iills the annular region 16 is comprised of acontinuous spiral of corrugated copper foil. This forms, in eifect, asshown in Fig. 4, a system of small diameter copper straws or tubes 20which completely till the annular region 16. The corrugated foil member13 may be fabricated from OFHC copper sheet 0.003 inch thick and 6inches wide. The corrugations 22 in the sheet 18 may be obtained bypassing a tlat sheet of material between two matching sections of gearstock backed by 1 inch diameter cold-rolled steel rolls. lt is desirableto form the corrugations 22 at an angle with respect to the direction ofthe copper sheet to create a more devious path through the member 1S asshown in Fig. 2. The resulting material 18 has the corrugations 22approximately .025 inch deep and when rolled up forms passages 20approximately .050 inch diameter.

One limitation on the design of traps is the fact that any trap willreduce the effective speed of pumping and it is thereforeimportant touse a trap which shall have maximum practical conductivity. l have foundthat a cylindrical tubing 2.8 centimeters in diameter completely filledwith copper foil such as previously described has approximately the sameconductance as an unfilled tube of the same length l centimeter indiameter. For a reentrant trap whose interior tube 14 diameter isapproximately onethird of the outer diameter of the envelope 10, theconessere@ 3 ductance is reduced by only a factor of two through theintroduction of the copper foil member 18. The copper foil member 18represents an extensive surface area and a large effective ratio oflength to diameter for each passageway 22 through the member 18.Although I described only one structure for member 18, this invention isnot limited thereto. Itis only necessary that the material of the memberbe of high heat conductivity and suitable for withstanding bake out. Themember 18 may be of any configuration limited only in that it ispermeable to gas.

In the installation of the trap, as described within a vacuum system asdescribed in Figs. l and 2, the inlet opening to the trap consisting ofthe tubular member 14 is connected to the diusion pump side of thesystem while the outlet opening in the trap consisting of the pipe l? isconnected to the side of the system to be evacuated. It is thencustomary after the system is reduced to pressure of the order of -2 mm.mercury to place a pot 26 containing the refrigerant 28 at a level 30around the lower i portion of the envelope 10.

In the operation of the device, the oil or mercury molecules diffusefrom the pump into the inlet tubular member 14 and pass to the lowerregion of the envelope 10 and then diffuse or pass up through the copperfoil member 18 and are trapped within the trap before they can go intothe outlet pipe 12. As previously mentioned with respect to priordesigned traps, the dropping of the level of the refrigerant liquidresulted in the re-evaporation of the oil molecules condensed on thepreviously cold surfaces of the envelope 10 and the tubular member 14 asthe level of the refrigerant dropped. In my device the high thermalconductivity of the material within the annular region 16 insures thatthe entire trapping surface remains cool and is essentially independentof the normal variations in the level of the refrigerant liquid. This istrue in that regardless of the level 30 of the refrigerant 28, a portionof the member 18 will be below the level 30 of the refrigerant 28.

The use of the high thermal conductivity material 18 insures that theentire surface of the material will be held at substantially the sametemperature which is essentially dependent on the temperature of therefrigerant. Although the material 18 comprises essentially the entirearea onto which the vapors condense, there will` be a small portion onthe envelope 10 and the tubular member 14. Since the members 10 and 14are of glass material of poor thermal conductivity, the effectivecondensing surface will be limited to the surface below the level of therefrigerant 28. The trap insures that there will be no f re-evaporationof vapors from the members 10 and 14. The metal type trap has aparticular advantage for use with mercury type pumps. The mercury has agreater probability of sticking to a cold metallic surface than to acold glass surface. metals with which mercury amalgamates, such ascopper and silver. Thus the trap is uniquely suited as a cold trap forisolating Hg from adjoining containers or vacuum systems.

One of the unusual properties discovered of the trap described in Figs.l and 2 was found to be in its effectiveness for isolating the system tobe evacuated from pumps using an organic liquid such as octoil oroctoil-S, generally referred to as oil pumps, even whileV the trap isheld at room temperature. It was found that after a high temperaturebakeout or outgassing at a temperature of 400 to 500 C. for a period ofone to ten hours with a vacuum of pressure of 10-3 mm. mercury alongwith the rest of the system in the evacuating cycle that a low pressurewas obtainable with further pumping even without the refrigerant appliedto the trap. The trap may be baked out separately while on the system ata temperature of about 300 C. It is necessary to bake a charcoal trapout at a temperature of 700 C. and the pressure obtained was only of theorder of 10-8 mm. mercury. I have been This is especially true in thecase of l able to obtain a pressure of about 10"11 mm. mercury withoutthe use of any cooling medium around the trap described herein.

As a result of this feature, it is seen that a trap may alternatively bedesigned substantially in the straight through type form as shown inFig. 3. The device shown in Fig. 3 is comprised of an elongated tubularmember 32 having an inlet opening 34 and an outlet opening 36 atopposite ends of the member 32 and a high thermal con* ductivity typemember 38 positioned within the tubular member 32. The member 38 may beof similar structure to the member 18 previously described and is woundto lill the tubular member 32 between the openings 34 and 36.

it has been found possible to obtain and maintain a pre: of 'lO-11 mm.mercury for a week or more in a vacuum system isolated from the oil pumpby the held at only room temperature. The composition of the material ofthe member 18 or 3S utilized in a trap at room temperature must be ofsuitable melting point metal or alloy such as copper, an alloycomprising nickel, l5% chromium and 5% iron (such as that sold undertradename, Inconel), nickel, silver titanium, iron and bronze. Theoperation of the room temperature trap has been found to be especiallygoed with respect to copper and lnconel.v It is believed that thismaterial exhibits the property of preferential sorption for theimpurities coming from oil diffusion pumps when the surface is clean.The term clean is delined to mean desorbed of surface layers of gas.This is achieved by bake-out at temperatures above 300 C. for asufficient interval to remove said layers. The term Sorption is definedto indicate either the removal of gas and vapors by absorption oradsorption to distinguish from the condensing property of the cold trap.It also has been found that by using a metal that has a lightly oxidizedsurface and then reducing with hydrogen prior to bakeout substantiallyincreases the effectiveness of the trap and the length of time it willcontinue to capture oil molecules. This process produces amicroscopically irregular surface. It is found that after several daysof operation that the vacuum obtained of the order of 10-11 mm. mercuryis gradually reduced to the order of 10-8 mm. mercury depending on theamount of surface available in the material 18 or 38.

While I have shown my invention in two forms, it will be obvious tothose skilled in the art that it is not so limited but is susceptible ofvarious changes and modifications without departing from the spirit andscope thereof.

I claim as my invention:

1. In arhigh vacuum system, in combination with a high vacuum pump, ahigh vacuum vapor trap comprising a substantially evacuated envelopehaving therein an inlet and an outlet opening and a clean metallicmember composed of a substance which is substantially impervious to gasmolecules and which may be heated above 300 C. without melting, saidmember being in the form of a metallic corrugated spiral sheet so thatsaid member has a plurality of penetrating passageways, said membersubstantially iilling the region between said inlet and outlet openings.

2. In a high vacuum system, in combination with a vacuum pump, a highvacuum vapor trap comprising a substantially evacuated envelope havingtherein an inlet and an outlet opening and a clean metallic corrugatedspiral sheet member substantially lling the region between said inletand said outlet openings, said metallic members being composed of asubstance which is substantially impervious to gas molecules, saidmember having a plurality of penetrating passageways, said metal memberbeing conditioned by being baked out at a temperature of about 300 C.Without melting Within said vacuum system.

3. In a high vacuum system, in combination with an oil diffusion vacuumpump, a high vacuum vapor trap comprising an elongated substantiallyevacuated enclosure having an inlet opening and an outlet opening, meanswhereby oil is unavoidably diffused from said pump to said trap andmeans to remove said oil by sorption, said removing means comprising aclean metallic member positioned within said enclosure, said metallicmember being capable of preferential sorption of oil molecules from saidpump, said metallic member being composed of a substance which issubstantially impervious to gas molecules and which may be heated above300 C. without melting, said metallic member having a plurality ofpenetrating passageways and said member substantially filling the regionbetween said inlet and outlet openings.

4. In a high vacuum system, in combination with an oil diffusion vacuumpump, a high vacuum vapor trap comprising an elongated cylindricalsubstantially evacuated envelope having an inlet opening and an outletopening positioned at opposite ends of said envelope, means wherebyvapors are unavoidably diffused from said pump to said trap and means toremove `said vapors by zsorption, said removing means comprising aspiral wound corrugated metallic foil member positioned within saidenvelope, having a plurality of penetrating passageways, andsubstantially lling the region between said inlet and outlet openings,said foil member being characterized in that the surfaces are clean andare capable of preferential sorption of vapors from said pump, said foilmember being composed of a substance which is substantially imperviousto gas molecules and which may be heated above 300 C. Without melting.

5. In a high vacuum system, in combination with a high vacuum pump, ahigh vacuum vapor trap comprising a substantially evacuated envelopehaving an upper portion and a lower portion, an inlet tubular memberconnecting with said upper portion of :said envelope and extendingdownward through said envelope to said lower portion, said tubularmember forming with the wall of :said envelope an annular region, anoutlet opening in said upper portion of said envelope, means wherebyvapors are unavoidably diffused from said pump into said vapor trap, andmeans to remove said vapors by sorption, said removing means comprisinga clean metallic member being composed of a substance which issubstantially impervious to `gas molecules, which may be heated above300 C. without melting, and which has a high thermal conductivity, saidmetallic member having a plurality of penetrating passageways, saidmetallic member substantially filling said annular region.

6. In a high vacuum system, in combination with a high vacuum pump, ahigh vacuum vapor trap comprising a substantially evacuated envelopehaving an upper portion and a lower portion, an inlet tubular memberconnecting with said upper portion of said envelope and extendingdownwardly through said envelope to said lower portion, said tubularmember forming with the wall of said envelope an annular region, anoutlet opening in said upper portion of said envelope, a clean metallicmember substantially filling said region, said clean metallic memberbeing composed of a substance which is substantially impervious to gasmolecules, which may be heated above 300 C. without melting, and whichhas a high thermal conductivity, said metallic member being in the formof a corrugated spiral sheet so that said member has a plurality ofpenetrating passageways.

7. In a high vacuum system, in combination with a high vacuum pump, ahigh vacuum cold vapor trap comprising a substantially evacuatedenvelope having an upper portion and a lower portion, said lower portionbeing adapted to be immersed in a cooling medium, an inlet tubularmember connecting with said upper portion ol said envelope and extendingdownwardly through said envelope to said lower portion, said tubularmember forming with the wall of said envelope an annular region, anoutlet opening in said upper portion of said envelope, means wherebyvapors are unavoidably diffused from said pump to said trap, and meansto remove said vapors comprising a clean metallic member substantiallyfilling said region, having an extensive surface area and having aplurality of penetrating passageways, said clean metallic member beingcomposed of a substance which is substantially impervious to gasmolecules, which may be heated above 300 C. without melting, and whichhas a high thermal conductivity.

8. An evacuating system comprising, in combination, a chamber to beexhausted, an organic liquid diffusion vacuum pump, a conduit meansforming a passage between said chamber and said pump, whereby organicliquid vapors are unavoidably diffused from said pump along said conduitmeans and means for removing said organic vapors by sorption, saidremoving means cornprising a high vacuum vapor trap, said trap includinga substantially evacuated envelope having an inlet opening and an outletopening, said trap also including va clean metallic member capable ofpreferential sorption of organic vapors from said pump, said metallicmember being composed of a substance which is substantially imperviousto gas molecules and which may be heated above 300 C., without melting,said metallic member having a plurality of penetrating passageways andsubstantially lling the region between said inlet opening and saidoutlet opening, said metallic member having an extensive surface area.

9. In a high vacuum system, in combination with a high vacuum pump, ahigh vacuum vapor trap comprising a substantially evacuated envelopehaving an inlet opening and an outlet opening and a clean metallicmember of extensive surface substantially lling said envelope, saidsurface area being conditioned by the steps of oxidation, reduction andcleaning, said metallic member being in the form of a corrugated spiralsheet so that said member has a plurality of penetrating passageways,said member 'being composed of a substance which is substantiallyimpervious to gas molecules and which may be heated above 300 C. withoutmelting.

References Cited in the le of this patent UNITED STATES PATENTS1,535,157 Hughes et al Apr. 28, 1925 1,644,828 Guibert Oct. 11, 19272,187,470 Collins Jan. 16, 1940 2,317,814 Schuchmann et al Apr. 27, 19432,671,337 Hulsberg Mar. 9, 1954

